It is already known to provide reliable storage and retrieval of large volumes of digital data, such as computer data, in particular by means of the DDS (Digital Data Storage) format defined in ISO/IEC Standard 10777:1991 E.
In the DDS system an elongate recording media comprising tape coated with a magnetic medium is moved by a motor-driven capstan along a path wrapped partially around a transducer comprising a rotating drum carrying one or more electromagnetic heads. The plane of rotation of the drum is disposed at an angle to the plane of movement of the tape, so that each head traverses the tape along successive tracks extending across the width of the tape at an angle to its centreline.
Typically the precise location of each track is determined during recording of data on the tape by the position relative to the tape of write heads on the drum. To ensure optimum retrieval of data from the tape, it is desirable for read heads on the drum to follow paths across the tape which are at essentially identical positions to those followed by the write heads during data recording. This in turn requires control during data retrieval of relative motion between the tape and the drum, by controlling rotation of the drum and/or controlling movement of the tape by the capstan.
One method of providing such control is described, in the context of a helical scan video tape recorder, in U.S. Pat. No. 4,954,902. In the system described therein horizontal video synchronizing signals are recorded at multiple equally-spaced locations along each track. A position or head-exchange signal is generated during each revolution of the rotating drum, at a predetermined angular position. During playback (equivalent to data retrieval) this signal is used to generate pulses which are phase-locked to the position signal and the times of occurrence of which are compared with those of respective synchronizing signals detected on the tape. The capstan is controlled to provide relative motion of the tape and the head drum so that a predetermined tracking time period (half the interval between pulses) is maintained between occurrence of each pulse and detection of each synchronizing signal.
Although this system provides acceptable results and is in principle applicable to the DDS system, there is a practical problem in the case of data storage. Whereas video signals inherently contain a convenient series of regularly-spaced synchronization signals (the video horizontal sync signals), computer data do not, and it is not necessarily convenient to insert corresponding tracking signals at regularly spaced points. Furthermore, the system described in U.S. Pat. No. 4,954,902 assumes that the synchronization signals (and thus the tracks) are always spaced from the edge of the tape by a predetermined distance; however, in practice the position of the tracks relative to the edge of the tape is not very tightly controlled, and this can lead to substantial variations between different recordings in the time interval between occurrence of the drum position signal and occurrence of a tracking signal. Thus, if a tape contains several data recordings made at different times, there can be an abrupt change in the required tracking time period at each transition between recordings. The system of U.S. Pat. No. 4,954,902 does not address this problem.
It is an object of this invention to provide a method and apparatus for controlling the motion of media in, for example, a DDS mechanism using a tracking time period to control tape movement, and which can accommodate such changes.